A resident of the Sierra Nevada foothills questions the possible reasons for the recent spate of Lake Tahoe quakes.
By Lester Lubetkin
We live outside of the small community of Placerville, Calif., situated in the Sierra Nevada foothills halfway between Sacramento and Lake Tahoe. Lately, we have been jolted by earthquakes that are centered in the graben that forms Lake Tahoe. By jolted, I mean it feels somewhere between someone slamming the front door or driving their car into the garage wall. It is a sharp “crack” — in most cases I have not felt the subsequent rolling as the shockwaves pass by, although my more attuned wife does often sense the prolonged motion.
Over the last several weeks there have been at least three or four earthquakes that were strong enough for us to notice them. We often go right to the U.S. Geological Survey (USGS) earthquake website to see where the epicenters are and what the magnitude is. Turns out the earthquakes we have been feeling range in magnitude between 2.5 and 4.2, and actually, since May 6, there have been 10 earthquakes located within Lake Tahoe and another six between Truckee, northwest of the lake and Markleeville, to the southeast. These earthquakes feel very different from the large but more distant San Francisco Bay Area earthquakes we sometimes feel. Those tend to be more rocking and rolling and can be felt for a much longer time period.
Are these earthquakes causing the graben that holds Lake Tahoe to become deeper? Or is this series of earthquakes related to Walker Lane, which helps take up some of the “leftover” right-lateral displacement between the Pacific Plate and the North American Plate that is not already accommodated by the San Andreas Fault? Or neither?
It seems that we have three possible explanations for seismicity along the eastern side of the Sierra Nevada: (1) There is the western limit of the Basin and Range extension, leading to the graben that has set the stage for the formation of Lake Tahoe; (2) There is the uplift of the west-tilted Sierra Nevada range (although recent studies have suggested this may not be as recently rapid as previously thought); and (3) The complex motion of blocks within the 65-mile-wide Walker Lane, accommodating the relative motion between the North American and Pacific plates. Or, are each of these stress fields superimposed upon each other, so that one can’t really distinguish each from the others?
So again, I go to the USGS earthquake website to see if the focal mechanisms (those funny beach-ball figures) can help to answer which of these events is happening. Interestingly, the focal mechanisms for several of the earthquakes show that the relative motion along the faults has been right-lateral strike-slip, with only a slight vertical component. This suggests that these earthquakes are part of the Walker Lane seismic activity, which may over millions of years cause the Gulf of California to extend northward into Southern California and shift the Sierra Nevada northward. But some of the other earthquakes show more oblique slip, fitting into either graben extension or Sierran uplift.
We can look at some of the physical conditions to see if they are consistent with the focal mechanisms. Waldemer Lindgren, back in 1896, recognized that the South Fork of the American River is truncated (or beheaded) along the fault on the western side of the Lake Tahoe graben. More recent studies by Richard Schweickert and others have shown that the graben started to form about 3 million years ago, and that before that time, the South Fork American River extended eastward. And in fact, the eastern extension can still be found forming the broad valley that holds Luther Pass. This eastern portion of the South Fork American River creates a wonderful piercing point, allowing us to constrain the amount of horizontal motion that might have occurred over the last 3 million years in this portion of the Lake Tahoe graben. It appears that there may be as much as 1.5 to 2 miles of right-lateral offset of this ancient channel of the South Fork American River. Very roughly, that would equate to 0.03 to 0.04 inches per year over the last 3 million years (0.075 to 0.1 mm/yr).
But then, what about the oblique slip component? I’d sure love to hear from others as to observations and ideas. How do we unravel the stories in these regions with complex, superimposed stress regimes?
Editor’s note: Lester Lubetkin, a long-time resident of the Sierra Nevada foothills, retired from the Eldorado National Forest where he served as a geologist and recreation manager and now volunteers in interpretation, forest restoration and community fire resiliency.
References
Lindgren, W. (1896). Pyramid Peak Folio, California. Geological Survey (United States).
Schweickert, R. A. (2009). Beheaded west-flowing drainages in the Lake Tahoe region, northern Sierra Nevada: Implications for timing and rates of normal faulting, landscape evolution and mechanism of Sierran uplift. International Geology Review, 51(9-11), 994-1033.
Wakabayashi, J. (2013). Paleochannels, stream incision, erosion, topographic evolution, and alternative explanations of paleoaltimetry, Sierra Nevada, California. Geosphere, 9(2), 191-215.
